Recently, a semiconductor device including a thin-film transistor (TFT) and a thin-film diode (TFD) on the same substrate and electronic devices with such a semiconductor device have been developed. It was proposed that such a semiconductor device be fabricated by making the respective semiconductor layers of the TFT and TFD of the same crystalline semiconductor film on the substrate.
Patent Document No. 1 discloses an image sensor including, on the same substrate, a photosensor unit that uses a TFD and a driver that uses a TFT. According to Patent Document No. 1, the respective semiconductor layers of the TFT and TFD are obtained by crystallizing an amorphous semiconductor film that has been deposited on a substrate.
If the TFT and TFD form integral parts of a single semiconductor device on the same substrate in this manner, not just the overall size of the semiconductor device but also the number of required parts can be reduced, thus cutting down the cost significantly. On top of that, products with new functions, which could not be achieved by conventional combinations of parts, can also be provided.
On the other hand, Patent Document No. 2 discloses a technique for using the same semiconductor film of amorphous silicon to form a TFT of crystalline silicon (which will be referred to herein as a “crystalline silicon TFT”) and a TFD of amorphous silicon (which will be referred to herein as an “amorphous silicon TFD”) on the same substrate. Specifically, a catalyst element that promotes the crystallization of amorphous silicon is added to only a portion of the amorphous silicon film on the substrate to be the active region of the TFT. After that, a heat treatment process is carried out, thereby obtaining a silicon film, of which only the portion to be the active region of the TFT has been crystallized but a portion to be the TFD remains amorphous. By using such a silicon film, the crystalline silicon TFT and the amorphous silicon TFD can be fabricated on the same substrate more easily.
Furthermore, according to Patent Document No. 3, the same semiconductor film (of amorphous silicon) is used to make a photosensor TFT functioning as a photosensor and a switching TFT functioning as a switching element. By making the silicon film that defines the channel region of the photosensor TFT thicker than the silicon film that defines the source and drain regions thereof or the active region of the switching TFT, the sensitivity of the photosensor is increased. According to that patent document, to make the thicknesses of the respective silicon films of those TFTs different from each other, when a photolithographic process is carried out to divide the amorphous silicon film into a number of islands, the amorphous silicon film has its thickness partially reduced by half exposure process that uses a gray tone mask. Patent Document No. 3 also discloses that by irradiating the amorphous silicon film with a laser beam, those thinned portions of the amorphous silicon film (i.e., portions to be the source and drain regions of each photosensor TFT and the portion to be the active region of each switching TFT) are crystallized but the other non-thinned portion (i.e., the portion to be the channel region of the photosensor TFT) is left amorphous.